Method of polishing metals



Patented Apr. 6, 1943 Charles L. Faust, Columbus, Ohio, assignor toBattelle Memorial Institute, Columbus, Ohio, a

corporation of Ohio No Drawing.

Application November 23, 1938,

Serial No. 241,976

3 Claims.

This invention relates to a method of polishing metals. to impartthereto a lustrous,- mirrorlike polish.

It has heretofore been common practice to polish metals by a mechanicaloperation, either on automatic polishing and bufling machines, ormanually, or by a combination of automatic and manual operations. Inaddition to the considerable cost of mechanical polishing and bufilngoperations, particularly where the final bufllng operations have to beperformed manually, there are some drawbacks in the mechanical polishingof metals which are inherent because of the properties of the metalsthemselves. Some metals, such as stainless steels, for instance, arerelatively poor conductors of heat, in consequence of which the highspeeds of the polishing or bufflng wheels tend to burn the surfacesthereof. This greatly limits the speed at which the pol- 1shingoperations may be carried out.

Furthermore, in the mechanical polishing of metals, the surfaces exhibita tendency to pile, with the result that the polished surfaces soproduced are cold worked and strained. Such surfaces are characterizedby the presence of a layer of amorphous or pseudo amorphous materials.Also, in any commercial polishing process, the surface will be found tobe marked by microscopic scratches.

Mechanical polishing has the further objection that it involves aconsiderable amount of comparatively highly paid labor. In the case ofstainless steel, for example, mechanical polishing is one of the mostexpensive steps in the finishing operations, so that polished stainlesssteels have heretofore sold for a price which is very high in comparisonwith the material cost. The cost of mechanical polishing is also a majoritem in the manufacture of chromium plated articles, particularlyarticles of irregular shape that must first be polished, then nickelplated and finally buffed before the application of the chromium plate.

While electrolytic methods for the treatment of metals and alloys toclean or etch the surfaces of the metals are well known, such methodshave been directed mainly to those purposes and not to the production ofpolished surfaces. The patent to Burns et al. No. 1,658,222, datedFebruary 7, 1928, for instance, refers to the anodic electrocleaning offerrous metals preparatory to nickel plating and alleges that a smooth,polished and uniformly etched surface may be produced by anelectrochemical cleaning action. By following the procedure disclosed inthis patent, however, I have been unable to obtain a brightly polishedsurface but merely a surface that is somewhat brighter than thatobtainable by the usual pickling processes.

Similarly, although the Blaut et a1. Patent 2,115,005, dated April 26,1938, purports to describe an electrochemical cleaning process that willproduce a burnished surface, I have been unable to obtain by followingthe procedure therein disclosed such a highly lustrous polish as isreadily produced by the use of the electrolytic solution of my presentinvention.

I have now found that a superior polishing effect may be produced morereadily and at a lower cost than was heretofore possible through the useof an electrolytic solution consisting essentially of sulfuric acid andan organic compound capable of forming a complex ion with a metalliccomponent of the metal or alloy undergoing treatment. Sulfuric acidalone is not sufficient to produce the polishing effect characteristicof my process, but this effect can be obtained by the use in conjunctionwith sulfuric acid of any one of a wide variety of organic materials,such as glycerol, glycyrrhiza and soluble glycols.

It is therefore an important object of my invention to provide anelectrolytic solution for use in the anodic treatment of metals toimpart thereto a highly lustrous surface directly and without requiringany subsequent mechanical buffing or polishing operations.

It is a further important object of my invention to provide a method forthe electrochemical treatment of metals to produce thereon surfaces thatare highly lustrous and free from the scratches and piled layerscharacteristic of mechanically polished surfaces of metals, such asstainless steels.

It is a further important object of this invention to provide anelectrolytic solution consisting essentially of sulfuric acid and anorganic compound capable of forming a complex ion with iron, whichsolution may be used in the polishing of metals'to impart thereto highlylustrous surfaces at a cost considerably less than that entailed bymechanical operations, while at the same time producing surfaces havingrelatively superior characteristics to those obtainable by mechanicalpolishing or bufiing operations.

Other and further important objects of this invention will becomeapparent from the following description and appended claims.

The method to which this invention pertains involves making the metal tobe polished an anode in an electrolytic bath of suitable composition andpassing a current of sufllcient density and for a sufficient length oftime to produce a high lustre or polish on the metal. By employing theelectrolytic solution'of my present invention, results are obtained thattranscend the mere electrolytic cleaning process, in that there isproduced a highly lustrous or polished surface. This high lustre is animportant feature of my invention and one that sharply distinguishes itfrom prior art finishes produced in the electrolytic cleaning of metals.

The electrolytic solution that I have found to possess the most generalapplicability comprises a mixture of sulfuric acid and an organiccompound capable of forming a complex ion with a metallic component ofthe metal or alloy undergoing treatment, such organic compoundsincluding a wide variety of organic materials, such as glycerol, solubleglycols, ammoniated glycyrrhizin and glycyrrhiza extract or liquor.Certain alcohols, such as methanol, may be a-dvantageously used inconjunction with mixtures of sulfuric acid and any one or more of theforegoing organic compounds. The electrolytic solutions so produced areeffective in producing lustrous surfaces when employed in accordancewith the method of this invention in the treatment of iron and itsalloys, nickel and its alloys and copper.

The formation of highly polished and lustrous surfaces, which is adistinguishing feature of my invention, is undoubtedly associated withthe presence of a polarizing film over the surface of the metal duringthe process of anodic dissolution. The nature of this film is such thatselective attack of the various phases present in the metal or alloy isminimized' Anodic dissolution apparently takes place at a relativelyhigh rate at high anodic polarization value, with the result that theanodic dissolution of the metal acts to level the crystal surfacesthereof and to produce a mirror-like finish. These conditions do notprevail in the simple electrolytic cleaning treatments known to theprior art.

In the electrolytic solution of my invention the sulfuric acid contentmay be widely varied, as from 1 to 90%, but is preferably in excess of50% and not over 90%. Where another mineral acid, such as orthophosphoric acid, is mixed with the sulfuric acid and organic compound,the total acid concentration should likewise lie within the range of 50to 90%. The higher concentration of total acid is more particularlyrequired, when water is present, if a good polish is to be obtained,while the lower concentrations of acid may be satisfactorily used in anelectrolytic solution containing practically no water. An example of thelatter case is an electrolytic solution comprising 94% of glycerol, 1%of sulfuric acid and 5% of ammonium chloride.

In order to obtain the best results, in a reasonable length of time, itis preferable to use relatively high current densities of the order ofmagnitude of from 300 to 1,000 amperes per sq. it, although it will beunderstood that lower current densities may be employed with consequentprolongation of the time of treatment.

'Higher current densities than indicated by the foregoing range may alsobe used, although such high current densities are necessarily morecostly because of the higher cost of electrical energy. The length oftime to-efiect the desired results depends upon the magnitude of thecurrent densities employed and to some extent upon the of its surfaceinitially. Rough surfaces, of course, require a longer time to polishthan relatively smooth ones.

The treatment depends to some degree on the previous heat treatment, ifany, of the metal. The condition of the various phases present in themetal influences the character and degree of the attack upon the grainboundaries, and in some cases it is necessary to increase the currentdensities in order to obtain satisfactory polishes.

The following examples will serve to illustrate preferred compositionsof my electrolytic solutions and the conditions under which suchsolutions produce satisfactory polishes. Unless otherwise stated, allpercentages will be understood to indicate percentages by weight, ratherthan by volume.

EXAMPLE 1 Bath composition With the above bath, excellent polishes areobtained on 18-8 chromium-nickel stainless steel,

particular metal to be polisheda'nd the character when the stainlesssteel is made the anode therein at current densities of 500 amperes persq. ft. and the treatment carried out for a period of 1 to 12 minutes.This time suffices to produce a very brilliant surface at a bathtemperature of to 175 F.

EXAMPLE 2 Bath, composition I Per cent Sulfuric acid 50 Glycerol 40Water 10 This bath, when employed with current densi-- ties of 300 to1000 amperes per sq. ft. for the anodic treatment of 18-8 and 24-12chromiumnickel, and straight chromium stainless steel, develops a verybrilliant surface. The time required at temperatures of to 200 F. rangesfrom 3 to 9 minutes, depending somewhat upon the current densities usedand the original state polishing of 18% straight chromium stainlesssteel at current densities of around 1000 amperes per sq. ft. r

. EXAMPLE 4 Bath composition Percent Sulfuric acid 1 Ammonium chloride 5Glycerol 94 This bath produ es a -good polish on Monel metal at currentdensities of 750 to-1000 amperes per sq. ft. if the treatment is carriedout for 1 minute at 86 F.

EXAMPLE Bath composition Per cent Sulfuric acid 33 Glycerol 33 Water 34A bath of the foregoing composition produces a satisfactory polish onnickel and German silver when they are treated anodically therein atcurrent densities of 1000 to 2000 amperes per sq. ft. for seconds at 96F.

ExAMPLE 6 Bath composition Per cent Sulfuric acid 50 GlycerolHydrochloric acid 2 Water Balance The foregoing bath produces a surfaceof high lustre on ordinary steel at 50 amperes per sq. ft. for 2 hoursat F. v

From the foregoing description of my invention, it will be apparent thatI have provided a novel and effective way of producing highly lustrouspolishes on stainless steels, nickel and its alloys, and plain carbonsteels. My method avoids the disadvantages above enumerated of the oldmethod of mechanical polishing and enables the production of metalarticles having surfaces that are free from mechanical strain, draggingand piling," and which are superior to those obtainable by mechanicalpolishing methods.

This application is a continuation-in-part of my application Serial No.218,388, filed July 9,-

wide range without departing from the principles of this invention andit is, therefore, not the purpose to limit the patent granted hereonotherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. The method of electropolishing metals selected from the groupconsisting of iron and its alloys and nickel and its alloys and Germansilver which comprises passing an electric current from the metal as theanode through a solution consisting of about sulfuric acid and 40%glycerol by weight, the balance being largely water, the current beingof suflicient density and being continued for a sufiicient length oftime to efiect the polish on said metal.

2. The method of electropolishing a metal or alloy selected from thegroup consisting of iron and its alloys, nickel and its alloys andGerman silver, which comprises passing an electric current from a metalor alloy selected from the group named as the anode through a solutionof about 33% sulfuric acid, 33% glycerol and 34% water, all percentagesbeing by weight, and continuing to pass said electric current while of asufllcient density for a sufiicient length of time to efiect the polishof said metal or alloy.

3. The method of electro-polishing a metal or alloy selected from thegroup consisting of iron and its alloys, nickel and its alloys, andGerman silver, which comprises passing an electric current from themetal or alloy selected from the group named as the anode through asolution of from about 33 to 50% sulfuric acid and from about 33 to 40%glycerine by weight, the balance being largely water, and continuing topass such electric current while of a suflicient density for asuflicient length of time to affect the polish of said metal or alloy.

CHARLES L. FAUST.

